I used to teach guitar to people to; my advise for all beginners is to practice every day for 3 weeks about 10 minutes a day, even if it hurts a bit in the beginning and after 3 weeks you will have no more problem playing half an hour or more, this advise worked for all the students I had from ages between 10 and 65 years old. If you play in a cover band at a party, you will likely play for 4 hours and more and that is hard work, believe me, I've been there.

Herman